The present embodiments relate to contrast agent destruction or imaging with ultrasound. In particular, medical diagnostic ultrasound destroys, with reduced biological effect, contrast agents.
Perfusion assessment uses contrast agents assisted ultrasound imaging of various organs. Contrast agents are typically microbubbles that may be destroyed by ultrasound energy. For perfusion assessment, an ultrasound destruction frame cleans-up or destroys microbubbles before imaging microbubble accumulation (wash-in) in the object of interest over time. Employing transmit pulses with higher peak-negative pressure and lower frequencies more likely destroys more microbubbles.
However, the higher pressure and lower frequency may cause biological effects since some of the bubbles grow bigger before breaking. Negative transient biological effects on myocardial function, coronary perfusion pressure, and microvascular integrity can occur during ultrasound contrast agent imaging within the normal FDA diagnostic mechanical index (MI) and thermal limits. For example, areas of epicardial hemorrhage appear in rabbit hearts exposed to contrast agents and ultrasound energy with a MI of 1.6 at harmonic imaging frequencies of 1.8 MHz/3.6 MHz. Micrographs show capillary rupture, extravasation of red blood cells into interstitial space and distribution of peroxidase activity in the myocardium covered by the ultrasound beam elevation and exposed to the MI of 1.6. Micrographs show normal microscopic appearance and an absence of significant peroxidase activity in myocardial regions located outside the beam elevation. Ultrasound with a 1.6 MI without contrast agents rupture much less than 1% of capillaries, but with contrast agents may rupture over 3% of capillaries. Ultrasound at 1.0 MI with contrast agents may rupture over 1% of capillaries.
To avoid ruptures or other biological effects, contrast agents are imaged with low MI, such as transmitting ultrasound energy with less than 0.6 MI. U.S. Pat. Nos. 6,494,841 and 6,632,177, the disclosures of which are incorporated herein by reference, show techniques for low MI contrast agent imaging. However, the destruction of contrast agents may be desired or unavoidable in some ultrasound imaging methods.